4.1

You can measure angles in radians.

One Radian is the measure of a central angle that intercepts an arc s equal in length to the radius r of the circle.

Complementary angles are angles that add up to 90 degrees.

Supplementary angles are angles that add up to 180 degrees.

To convert degrees to radians, multiply degrees by (pie)rad/180.

To convert radians to degrees, multiply radians by 180/(pie)rad.